Chemical vapor deposition (CVD) systems normally employ a chamber in which gaseous chemicals react. From these reactions, a substance is deposited on a surface of a workpiece e.g., a semiconductor wafer or dielectric plate or metal sheet to form dielectric, conductor, and semiconductor film layers that constitute an integrated circuit, for example. (Throughout, most of the remainder of this document, the workpiece is assumed to be a semiconductor wafer but it is to be understood that the invention is applicable to other types of workpieces.) In a chemical vapor deposition system, a process gas is injected into the plasma chamber in which a plasma is formed. Due to the ion bombardment within the plasma of the process gas (SiH.sub.4 (silane), SiF.sub.4 for example), silicon is deposited on a wafer which has been previously placed on a chuck in the chamber.
The chuck uses electrostatic force, for example, to hold the wafer securely in place during its processing. Since the workpiece covers the chuck during the processing of the semiconductor wafer, deposition of materials., e.g. silicon, onto the wafer bearing surface of the chuck is substantially prevented. However, in practice, a wafer may occasionally slide on the chuck, in which case a portion of the wafer bearing surface is exposed during the deposition process, causing deposition of material on the chuck wafer bearing surface. Alternatively, over time, deposition material may seep in under a wafer and leave a deposit on the chuck wafer bearing surface.
The presence of deposition material on the wafer bearing surface creates problems in the wafers that are manufactured in production runs. For example, deposition on the wafer may not have uniform thickness throughout the wafer as a result of hot spots created by the deposited material on the chuck wafer bearing surface.
Since creating deposition on wafers in a production run normally must have uniform thickness within precise tolerances, deposition on a chuck wafer bearing surface is a problem that must be corrected before additional wafers are processed. In the prior art, the process of removing the deposition from the chuck involved opening the sealed process chamber, removing the chuck, and cleaning the chuck with some material, such as hydrofluoric acid. This is a very expensive process since it shuts down the semiconductor manufacturing machine for a large number of hours. Time is required both to clean the chuck, as well as to re-prepare the chamber for continued processing. Since the chamber is a high vacuum chamber, it may take six or more hours to return the chamber to its operating condition. Also, if a chamber is opened for cleaning, a conditioning run of approximately 75 wafers must be executed before the processing can again be performed on production wafers.
It is known to remove deposition material (such as SiO.sub.2 or flourine-doped SiO.sub.2) in a sealed chamber by injecting a cleaning gas (such as NF.sub.3) and then applying RF power to the chamber. The walls are cleaned of oxide deposition. This chamber cleaning is normally performed periodically during a production run, such as after every five wafers have been processed. In this known process, however, the chuck is kept covered since prolonged NF.sub.3 exposure may damage the chuck and degrade the clamping force of the chuck. Another reason for covering the chuck in the prior art method is that temperature probes used in the chuck to measure the temperature of the wafer or for calibrating have been black body temperature probes that are damaged by exposure to NF.sub.3 and plasma. As the chuck is covered during the cleaning of the process chamber, the only method of cleaning the deposition material from the chuck according to the prior art has been to open the chamber and remove the chuck. As stated earlier, this is a very expensive and time-consuming process.